On Tuesday 29 April 2008 03:06:28 Bruggeman Christophe wrote:

I have recently collected EXAFS spectra of uranium on a FeS2 surface. Using principal component analysis of the XANES and k3-weighted EXAFS spectra, I have found that there are two uranium species which compose the spectra. As a first tentative guess, I believe these two uranium species are uraninite (UO2(c)) and a uranyl species. I would like now to fit the fourier transform functions (real parts and magnitudes) using the theoretical paths and path degeneracies created by feff, and use the amplitude reduction factor S02 as a fitting parameter to derive the relative amounts of the two uranium species in my samples.

Normally, this S02 is taken as a constant (between 0.7 and 1.0), and the path degeneracies are fitted. So normally, S02 is not really a fitting parameter (some papers derive it even with theoretical functions). However, given the fact that S02 and N are completely correlated, I think it is justified to use this approach.

Hi Christophe, The use of S02 in EXAFS analysis and its correlations with N and other parameters are discussed at considerable length in the archives of this mailing list. Here is a good place to start: http://cars9.uchicago.edu/iffwiki/FAQ/FeffitModeling Also seaching google for "site:millenia.cars.aps.anl.gov s02" turns up several more useful posts. In short, you seem to be on the right track. S02 is completely correlated with N in a first-shell fit. If you are fitting multiple corodination shells, you might be able to disentangle S02 from the various N's. And, of course, if you know N (for instance, when you measure EXAFS on a well-described crystal), then you might be able to determine S02 from your data. A word of caution, though. There are lots of other things that can effect the amplitude of your EXAFS, including error in normalizing data, error in Feff's self-energy model, detector non-linearity, sample inhomogeneity -- pretty much every part of the experiment from sample prep through theory can introduce systematic uncertainty and inaccuracy in your determination of EXAFS amplitude. Consequently, one often throws an amplitude parameter at a fit to account for all those various systematic issues. Given that, once the experiment is finished, those systematic issues are probably unknowable and unfixable, a variable amplitude parameter may be your best bet for extracting meaningful results from your data. HTH, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

Dear Christophe, The one drawback with using S02 in the way that you describe is that in so doing, you are assuming the true value of SO2 is 1. If you have a good reference spectrum and your reference is a well-defined solid, you can fit the reference spectrum to its known structure and determine SO2 for the experiment. You could then define a parameter, x, in artemis that is the fraction of UO2 then (1-x) would be the fraction of uranyl. The coordination number of each UO2 shell would be multiplied by x and each uranyl shell by (1-x). For what it's worth, SO2 for uranium oxides using Feff7 always seems to be around 0.9 for us if I recall correctly. Sincerely, Wayne -- Wayne Lukens Scientist Lawrence Berkeley National Laboratory email: wwlukens@lbl.gov phone: (510) 486-4305 FAX: (510) 486-5596 Bruggeman Christophe wrote:

I have recently collected EXAFS spectra of uranium on a FeS2 surface. Using principal component analysis of the XANES and k3-weighted EXAFS spectra, I have found that there are two uranium species which compose the spectra. As a first tentative guess, I believe these two uranium species are uraninite (UO2(c)) and a uranyl species. I would like now to fit the fourier transform functions (real parts and magnitudes) using the theoretical paths and path degeneracies created by feff, and use the amplitude reduction factor S02 as a fitting parameter to derive the relative amounts of the two uranium species in my samples.

Normally, this S02 is taken as a constant (between 0.7 and 1.0), and the path degeneracies are fitted. So normally, S02 is not really a fitting parameter (some papers derive it even with theoretical functions). However, given the fact that S02 and N are completely correlated, I think it is justified to use this approach.

Can someone comment on this?

Many thanks in advance,

Christophe

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